Heretofore, when waste acrylic or polystyrenic resin was thermally decomposed to obtain a decomposition product, the process was performed in a heating vessel capable of being externally heated. However not only there was an unavoidable defect with such process in that thermal conduction from the heating vessel is surprisingly insufficient because of vacant spaces which are formed between the heating vessel and the raw material resins due to irregular shapes of the raw material resin but also there was the further unavoidable defect that the thermal conduction is further hindered due to accumulation and solidification of carbon and other solid decomposition residues and substances having high boiling points on the bottom of the heating vessel which are difficultly removed. Accordingly, instead of the aforesaid process, a process in which as a heat transfer medium, a substance which may be changed to a molten state at thermal decomposition temperature i.e. metal, polyolefinic resin or the like is introduced together with the raw material resin into the heating vessel to thereby heat the raw material resin to decompose, has been employed.
When an inorganic material such as a metal i.e. zinc, tin and lead or an alloy i.e. solder and Wood's alloy is used as the heat transfer medium, there is an advantage in that cracked gas is not contaminated with the heating medium because of low vapor pressure of the inorganic heat transfer medium in general. On the contrary, there is a disadvantage with the inorganic heat transfer medium in that the raw material resin floats on the surface thereof because the specific gravity of said medium is greater than that of the raw material resin. Further, while no decomposition residue is sedimented so that there is no chance for the decomposition residue to be solidified on the bottom of the heating vessel, nevertheless there are defects in that at room temperature, not only the decomposition residue is solidified with the inorganic heat transfer medium which is difficultly removed, but also there is a danger of the decomposition residue damaging the heating vessel due to formation of an alloy between the decomposition residue and the heating vessel.
When an organic material such as a polyolefinic resin is used as the heat transfer medium, the specific gravity of the organic heat transfer medium is, in general, less than that of acrylic resin so that the raw material acrylic resin is sedimented in the organic heat transfer medium and is efficiently decomposed with sufficient thermal conduction. In this case there is also an advantage in that the decomposition residue may be easily removed together with the organic heat transfer medium because of dispersion of the decomposition residue in the heat transfer medium. However there is a defect in that the cracked gas of the raw material resin is contaminated with a substance having a low boiling point which is produced as the result of degradation of a part of the organic heat transfer medium, even at a temperature below that at which degradation of the raw material resin commences.
For a relatively long time, it has been known that a monomer is recovered in high yield by thermal decomposition of acrylic resin or styrenic resin. And as the process of recovering a monomer from methacrylic acid ester which has been conventionally employed, there is a dry distillation process wherein methacrylic acid ester polymer is introduced into decomposition vessel which is to be directly heated at 300.degree.c.
However with this process there are defects in that a monomer is recovered in a colored state, a decomposition residue is accumulated on the inside walls of the heating vessel to cause reduction of the thermal efficiency of the heating vessel and a long time is required for removal of this decomposition residue.
The present inventors have completed the present invention as the results of their studies on the advantages and defects when the above inorganic and organic heat transfer medium are individually employed.